The present invention relates to semiconductor vertical cavity surface emitting lasers (VCSELs) . More particularly, the present invention relates to VCSELs which provide high efficiency single-mode operation.
A VCSEL is a laser produced using semiconductors in which an optically active material is positioned between two reflective layers. Typically, the optically active material can comprise materials such as gallium arsenide, indium gallium arsenide, or quaternaries such as aluminum indium gallium arsenide, indium gallium arsenide phosphide, or indium gallium arsenide nitride. The reflective layers are typically of a multilayered dielectric material or metallic material. These layered mirrors are referred to as Bragg stacks. One of the mirrors is partially reflective as is typical of lasers such that coherent light built up in the resonating cavity between the two mirrors can pass out of the device.
Lasers have found many applications in modern technologies. In general, lasers are structures which require optical confinement of photons which are stimulated by pumping electrons into the device. Carrier confinement can be achieved by varying the resistivity of the materials between the electrical contacts and the active regions. Optical confinement is provided by employing materials having different refractive indexes which act as mirrors.
The Vertical Cavity Surface Emitting Laser (VCSEL) is currently a well established device for a variety of applications. The emission from this laser is normal to the plane of the device, and therefore, the device may be processed using standard semiconductor processing techniques, without the cleaving process required of edge-emitting devices. This manufacturing advantage is significant, since the devices can be tested before expensive packaging is performed. The beam profile is also symmetric (permitting more efficient coupling to fibers) and consists of a single longitudinal mode. These advantages make VCSEL devices less expensive to fabricate, reduce the complexity of the associated optics, and increase the performance and robustness of the entire system. For this reason, it is expected that VCSELs will be the laser of choice for an increasing number of electro-optic systems in the near future.
The structure of a VCSEL is comprised of an active region consisting of one or several quantum wells with appropriate barriers between the wells, spacer layers, and high reflectivity mirrors on either side of the active region. The mirrors consist of epitaxially deposited semiconductor material with an alternating low and high index, forming a highly reflective Bragg stack. The wavelength of emission of the laser is determined by the gain-bandwidth of the quantum wells and the cavity formed by the active region and the mirrors. There is a significant drawback to current VCSEL performance, however. Due to the small gain length, VCSEL power levels typically are small. In the current form of the VCSEL, the exciting electrical current passes through the Bragg stacks. However, since the alternating high and low index modulation is obtained with material of different bandgaps, heterojunctions result which increase the electrical resistance of the stack. Several techniques including grading of the composition, and varying the doping levels are used to reduce this resistance. A more recent innovation is to place an aperture of alumina (Al2O3), with a central region of either Al"khgr"Ga1-"khgr"As (with the mole fraction "khgr" being larger than 0.65) just above the active region so that the current funnels through this aperture center. This has resulted in VCSELs with lasing threshold currents of less than 1 mA.
A number of techniques have been used for forcing the devices to operate in single mode; these include spatial filtering (see R.A. Morgan et al., xe2x80x9cTransverse Mode Control of Vertical-Cavity Top-Surface-Emitting Lasersxe2x80x9d, IEEE Photon. Tech. Lett., Vol. 4, pp. 374-376, 1993), antiguiding techniques (see, Y. A. Wu et al., xe2x80x9cSingle-Mode Emission from a Passive Antiguiding Region Vertical Cavity Surface Omitting Laserxe2x80x9d, Electronics Lett., pp. 1861-1863, 1993), and external cavity techniques (see, B. Koch et al., xe2x80x9cSingle-Mode Vertical Cavity Surface Emitting Laser by Graded Index Lens Spatial Filteringxe2x80x9d, Apl. Phys. Lett.Appl. Phys. Lett., Vol. 70, pp. 2359-2361, 1997), and altering the top mirror structure (see, H. Martinson et al., xe2x80x9cTransverse Mode Selection in Large Area Oxide Confined Vertical Cavity Surface Emitting Lasers Using Shallow Surface Reliefxe2x80x9d, IEEE Photon. Tech. Lett., Vol. 11, pp. 1536-1538, 1999 and B. Koch et al., xe2x80x9cSingle Mode VCSELxe2x80x9d, Digest of the Conference on Lasers and Electro-Optics (CLEO 2000, San Francisco, Calif., pp. May 1, 2000). These techniques have resulted in VCSEL power output being limited to less than 5 mW.
One drawback with many existing VCSEL designs is their performance. VCSEL power levels for single mode devices are typically very small and increasing the device size results in multi-mode behavior with a significant increase in power.
Attempts to increase the power output of a VCSEL by increasing the diameter of the VCSEL have led to undesirable consequences. Currently, when the VCSEL diameter for the conventional structure is greater than about 7 xcexcm diameter, or in the oxide VCSELs, when the oxide central aperture diameter exceeds about 2.5 xcexcm diameter, the devices oscillate in several different transverse modes, and this results in the emission of light with as many wavelengths as there are modes. There are two major reasons for requiring single transverse mode operation. The first is that light from a multiple-transverse mode source cannot be focussed efficiently, so that coupling to devices such as single-mode fiber amplifiers is inefficient. The second reason is that the multiple wavelengths corresponding to the various transverse modes significantly compromise the operation of wavelength-sensitive systems such as single-mode optical fiber communication.
A vertical cavity surface emitting laser, includes a substrate, a lower mirror stacked adjacent the substrate, an active region operably coupled to the lower mirror stack, and an upper mirror operably coupled to the active region, opposite the lower mirror. The lower mirror, active region and upper mirror are arranged in a vertical stack to form a laser. A plurality of spaced apart regions of increased refractive index extend in a vertical direction at least partially through the vertical stack to promote single mode confinement in the stack. Other aspects include an etalon structure for tuning, an increase index of refraction region which is not circular in cross section, and a plurality of concentric rings around the core.